Write a sequence of instructions that transfers data from me

Write a sequence of instructions that transfers data from memory to an external I/0 device by using Channel 3 of the 8237. The memory area to be transferred is at location 20000H-20FFFH.

Solution

typedef struct
{
    char page;
    unsigned int offset;
    unsigned int length;
} DMA_block;

    Now, how do we find a memory pointer\'s page and offset?  Easy.  Use
the following code:

void LoadPageAndOffset(DMA_block *blk, char *data)
{
    unsigned int temp, segment, offset;
    unsigned long foo;

    segment = FP_SEG(data);
    offset  = FP_OFF(data);

    blk->page = (segment & 0xF000) >> 12;
    temp = (segment & 0x0FFF) << 4;
    foo = offset + temp;
    if (foo > 0xFFFF)
        blk->page++;
    blk->offset = (unsigned int)foo;
}